A vapor phase method for producing a glass preform for an optical fiber generally comprises jetting, from a burner, a mixture of a fuel gas, a glass raw material and optionally an additive for adjusting a refractive index of the produced galas preform, flame hydrolyzing the glass raw material in an oxyhydrogen flame to synthesize fine glass particles, depositing the fine glass particles on a rotating starting member to form a porous soot preform and then heating and sintering the porous soot preform to obtain a transparent glass preform. Usually, the fuel gas comprises a mixture of hydrogen and oxygen, the glass raw material comprises SiCl.sub.4, and the additive comprises GeCl.sub.4, POCl.sub.4 BBr.sub.3, etc. To increase a production rate and a size of the porous soot preform, two or more burners may be used in the vapor phase method.
Hitherto, when two or more burner are used in the conventional vapor phase method, it is rather difficult to control a bulk density of the fine glass particle mass deposited in the periphery of the soot preform and to increase yield.
Usually, the first center burner forms a flame so as to enclose a center portion of a surface of the soot preform on which the fine glass particles are being deposited (hereinafter referred to as "depositing surface"). However, the second burner(s) can heat only a part of the depositing surface since its flame cannot enclose the whole depositing surface as shown in FIG. 1, in which two burners are used for simplicity, and numeral 11 stands for a burner for forming a center portion of the porous soot preform, 12 stands for a burner for forming a peripheral portion of the soot preform, and 13 and 14 stand for regions which are heated by the burners 11 and 12, respectively. Since the soot preform is pulled up with rotation, some peripheral portions are not heated for a certain period of time so that the depositing surface is not sufficiently heated. Thus, the bulk density of the glass particle mass deposited in the peripheral portion becomes low, which tends to cause cracks.
When the flame stream is spread so as to increase the bulk density of the glass particles deposited on the periphery of the soot preform, convergence of the flame is decreased and the temperature of the center of the flame stream is lowered so that the production rate of the glass particles is deteriorated or the temperature of the fine glass particles is not sufficiently raised, which result in the decrease of the yield. When the flow rate of hydrogen is simply increased so as to raise the flame temperature and in turn the temperature of the depositing surface, interference between the burners induces the minimum point in a bulk density distribution as shown in FIG. 2, which causes the crack in the porous soot preform.
Thus, when two or more burners are used for synthesizing the fine glass particles in the production of the porous soot preform, the flow condition in the second burner(s) are so selected that the reaction rate of the glass raw material is increased, the temperature of the fine glass particles is raised, and the yield of the fine glass particles is improved and simultaneously that the flame formed by the burner(s) is controlled so as to obtain a stable distribution of the bulk density of the porous soot preform.
When SiCl.sub.4 is used as the glass raw material, the fine glass particles are synthesized according to the following equation: EQU SiCl.sub.4 +2H.sub.2 O.fwdarw.SiO.sub.2 +4HCl (I)
Although this reaction is exothermic, it should be promoted by heat evoluted by the oxyhydrogen flame since its reaction heat is as small as 24 kcal/mol. Thus, the synthesizing rate of the fine glass particles depends on the quantitiy of heat conducted from the flame and, consequently, the temperature distribution in the stream of the fine glass particles depends on the heat of the flame. Namely, when SiCl.sub.4 alone is used as the glass raw material, the flame formed by the burner contributes to the synthesis of the fine glass particles, heating of the synthesized fine glass particles and heating of the depositing surface of the porous soot preform. Therefore, conditions under which a large porous soot preform is stably produced with good yield and any one of the above described drawbacks is not encountered are very restricted.